Charged particle beam devices such as scanning and transmission or microprobe apparatuses, to quote only a few, are powerful instruments which permit the observation and characterization of heterogeneous organic and inorganic materials and their surfaces. In these instruments, the area to be examined is irradiated with a charged particle beam, which may be static or swept in a raster across the surface of the specimen. Depending on the specific application, the charged particle beam is more or less focused and the kinetic energy of the particles can vary considerably.
The types of signals produced when the charged particles impinge on a specimen surface include secondary electrons, backscattered electrons, Auger electrons, characteristic x-rays, and photons of various energies. These signals are obtained from specific emission volumes within the sample and can be used to examine many characteristics of the sample such as composition, surface topography, crystallography, etc.
Lately, attempts have been made to miniaturize charged particle beam devices. Several of these devices could then be grouped together to simultaneously examine larger areas of the specimen or they could be installed in process lines with tight space restrictions. Furthermore, since spherical and chromatic aberrations of particle beam devices scale proportional to their geometrical dimensions, as long as the potential remains constant, miniaturized devices would be able to deliver higher spatial resolution and high beam current in a given spot size.
In general, most of the present charged particle devices are between 0.5 and 1.2 meters high with an average diameter of about 15 cm -40 cm. Distinct from that, developers are aiming at producing beam devices which are smaller than 10 cm with an average diameter of about 4 cm. However, since modem charged particle beam apparatuses are complex technical instruments with sophisticated vacuum systems, alignment mechanism and electronic control units, their geometrical dimensions can not simply be shrinked proportionally, even though this is attempted wherever possible.
In low voltage applications, the performance of standard charged particle beam devices can considerably be increased by using a so called beam booster. It accelerates the charged particles within the optical column of the microscope to high kinetic energy and decelerates them before hitting the specimen. The deceleration step is mostly carried out when the charged particles pass the objective lens.
In general, the beam boost is achieved by surrounding the beam in the column with electrodes being shifted to a high accelerating potential. In microscopes with isolation valves positioned between the charged particle source and the specimen chamber or with any other conductive part arranged in the vicinity of the charged particle beam column, these parts are also shifted on the accelerating potential. The corresponding shielding electrodes, the valves or other conductive parts require insulation against the grounded column housing. Usually ceramic is used as insulating material. The end edges of the insulators are folded or provided with grooves to increase the creepage path. In miniaturized charged particle beam columns, however, this kind of isolation is not satisfactory.